The small human groups—like virtually every other mammalian omnivore— adapt to seasonal change, migrating to areas of optimal foraging and hunting at the appropriate time of year. Everyone agrees that the increasing human cranial capacity was accompanied by an increase in intellect— memory, symbolic manipulation, learning capacity, etc.
Somewhere in the history of this—and whether it started 2 mya or 20,000 ya is anybody’s guess—the mating structure changed. The largest anatomical differences between human and chimp brains are in the frontal lobes—those areas associated with executive functioning, evaluation, and reason.
Homo’s ability to transmit culture includes simple imitation but expands into symbolic instruction.
At some point Homo could communicate the idea “don’t do it that way, do it like XYZ” without ever physically demonstrating the “XYZ” behaviour.
Tool use—or at least the evidence of tool use—suggests that it developed into an art.
Spearheads were invented, bone instruments were fashioned to sew, pictorial drawings appear in caves, and some implements show evidence of engraving. The skeleton continued its gracile development and cranium capacity still increased to give its present day range of 1000 to 2000 cc, the average today being somewhere between 13 cc.
Still, we are stuck in the present and must do the best we can with the available data.
Most biologists suspect that humans and chimpanzees split off from a common ancestor as recently as 4 to 5 million years ago. One of the first evolutionary developments that distinguish human ancestors from chimps was upright posture.
The upper cranium of the skull expanded and assumed a more rounded shape, permitting brain size to increase from about 450 cc to between 8 cc.
Other evolutionary trends continued—the protruding jaw receded a bit, teeth became smaller, and height increased.